Many accurate quantitative models of homogeneous, bulk, carefully prepared microanalytical samples have been available for decades. These models are typically able to provide accurate compositional estimates from either wavelength dispersive or energy dispersive data sets based on comparison with standard reference measurements. The models which fall into two broad classes-ZAF and φ(ρz)-typically provide a series of factors to correct for differences in backscatter coefficients, stopping power, x-ray absorption, and x-ray fluorescence between a reference standard and an unknown sample. In addition to being developed for a very specific class of samples, each of these models was typically developed assuming specific (and unique to the author) values of certain fundamental input parameters such as the mean ionization potential and mass absorption coefficient. As our knowledge of these parameters has improved, it has become clear that many of the models were tuned to work well with what once were the best available parameter values but when more accurate values are introduced the effectiveness of the models actually decrease. The recent availability of new (and presumably better) theoretical and experimental values for many of these parameters suggests that it may be a good time to attempt to reconsider and refine these models with an eye towards rationalizing and simplifying the black art of quantitative microanalysis.

As a first step in this process, we have developed a sophisticated Monte Carlo model of electron transport and x-ray generation and absorption based on the latest and best available models for inelastic electron scattering, ionization cross-section and x-ray absorption. This model is unique in its flexibility to define complex sample geometries and to generate accurate synthetic spectra including contributions from both characteristic and continuum x-ray emission.

Using this model we intend to implement a multifaceted research program to (1) investigate more complex geometric classes of microanalytical problems, (2) investigate the validity of various different physical parameters that serve as inputs to the various correction models (such as mass absorption coefficients, φ [ρz] curves, and ionization cross-sections), (3) suggest where successful microanalytical correction models might be refined to handle a broader cross-section of problems with improved accuracy, and (4) develop a flexible correction model to handle particles using as input the morphologies measured using SEM-based automated particle analysis.

 

key words

Electron transport; Particle analysis; Quantitative microanalysis; Scanning electron microscopy;

Citizenship:  Open to U.S. citizens

Level:  Open to Postdoctoral applicants